Folders on External Disappeared...

A friend of mine has two 1TB hard drives (Western Digital and Seagate) which he plugged into a work computer and he said the folder icons changed to some window with blue and then disappeared. The drives say they have data, but can't see anything on them. He connected them to another computer that read them fine, but not anymore. CheckDisk doesn't find any errors and reads all the files in there. Could this be a virus? System seems to be clean of viruses, well at least the ones that can get picked up . There is no hidden files nor anything else that can be hiding all the files. Any ideas? If this topic is in the wrong place, please move it to the appropriate one, thanks!

He connected the first one, think it had some 600+ GB of data and he said that the folders started changing icons and he clicked one and they all vanished. He tried to connect it back to his laptop and can't see them anymore. The drive shows it has the space occupied. He then connected the 2nd drive to the work comp (don't know why) and it did the same thing. Now both drives have invisible data that can't be accessed anywhere. He messed with the folder options, but nothing and it doesn't make sense that no other computer sees the data now. Sounds really weird.